1. Field of the Invention
This invention relates to a transmission apparatus in a ring network.
2. Description of Related Art
Recently, with SDH (Synchronous Digital Hierarchy) technology being standardized, SDH networks have been introduced in many countries. In North America, SONET (Synchronous Optical Network) in compliance with the SDH technology is in the process of being introduced.
As such an SDH network, a ring network in which a plurality of SDH transmission apparatus are connected together like a ring is noticed as a network which can flexibly respond to the increasing scale of networks which constitute the current optical transmission systems.
As transmission apparatus for constructing such a ring network, known are those which can transmit a multiplexed signal on the order of 2.4 Gb/s (48-channel signal) and those which can transmit a multiplexed signal on the order of 10 Gb/s (192-channel signal), for example.
Meanwhile, as a signal transmission system employed in the above-mentioned ring network (ring type), known are two systems [UPSR (Uni-directional Path Switched Ring) system and BLSR (Bi-directional Line Switched Ring) system] incorporating therein means against line fault.
Here, the UPSR system (hereinafter simply referred to as UPSR) is mainly used, for example, in a local network or the like where all the information is once centralized at a single exchange. For example, in a ring network 100 shown in FIG. 34 in which four transmission apparatus 101 to 104 are connected together like a ring through two optical fibers 105-1 and 105-2, a multiplexed transmission signal (optical signal) is transmitted as follows.
For example, as shown in FIG. 35(a), in a normal state where no line fault occurs, when a transmission signal (signal of channel "1") from another line 101a connected to the transmission apparatus 101 is relayed and transmitted to the transmission apparatus 103 through the ring network 100, a line 101b passing through the transmission apparatus 102 and a line 101c passing through the transmission apparatus 104 are used together, so that the transmission signal is redundantly transmitted.
Here, the two optical fibers 105-1 and 105-2 connecting the transmission apparatus 101 to 104 together are set such that their respective optical signals, as multiplexed transmission signals transmitted through the ring network 100, are directed opposite to each other.
The optical fiber 105-1 is used for propagating the optical signal in the direction of arrow WE over the ring network 100; whereas the optical fiber 105-2 is used for propagating the optical signal in the direction of arrow EW over the ring network 100.
Also, the transmission apparatus 103 is equipped with a path switch 103A which selects, of the transmission signals respectively inputted via the two lines 101b and 101c, the one with a better quality having less errors and the like, for example, and relays thus selected signal to another line 103b [see FIGS. 35(a) and 35(b)].
Here, in the case where a fault occurs in the line 103a between the transmission apparatus 103 and 104, of the transmission signals transmitted from the transmission apparatus 101 to the transmission apparatus 103, the transmission signal transmitted through the transmission apparatus 104 is not inputted, whereas only the transmission signal transmitted through the transmission apparatus 102 is inputted.
Namely, as shown in FIG. 35(b), even in the case where a line fault occurs anywhere on the ring network 100 upon transmission of the transmission signal from the transmission apparatus 101, the path switch 103A of the transmission apparatus 103 can select the transmission signal passing through the transmission apparatus 102 on the faultless side and relay thus selected signal to another line 103b. As a result of such a redundant network configuration, the UPSR can improve the reliability of networks.
Each of all the transmission apparatus 101 to 104 constituting the ring network 100 has, with respect to each other, such a function as that of the transmission apparatus 101 upon signal transmission, and the function identical to that of the path switch 103A included in the transmission apparatus 103.
Further, in this UPSR, as shown in FIG. 36, a phase adjusting section 103B is disposed upstream the path switch 103A. As the phase of two transmission signals is adjusted by the phase adjusting section 103B before the transmission signal is selected by the path switch 103A, switching (uninterrupted switching) can be effected without hitting the signals.
Also, as shown in FIG. 37, a UPSR ring network 100-1 constituted by transmission apparatus 101-1 to 104-1, a UPSR ring network 100-2 constituted by transmission apparatus 101-2 to 104-2, and a UPSR ring network 100-3 constituted by transmission apparatus 101-3 to 104-3 can be connected together to form a large-scale UPSR ring network (virtual ring) 100-4 constituted by the transmission apparatus 101-1 to 104-1, 101-2 to 104-2, and 101-3 to 104-3. Accordingly, the UPSR characteristically has a high degree of freedom in design when constructing networks.
Meanwhile, the BLSR system (hereinafter simply referred to as BLSR), which is employed in communication networks between big cities or the like where there are a large amount of communications between stations, transmits multiplexed signals in systems shown in FIGS. 38(a) and 38(b), for example.
Namely, as shown in FIG. 38(a), in a normal state where no line fault occurs, when a transmission signal from another line 101a connected to the transmission apparatus 101 is transmitted to the transmission apparatus 103 through the ring network 100, unlike the above-mentioned UPSR, a backup channel is secured within the same route without using a signal from another route as backup line.
Specifically, the transmission signal to be relayed and transmitted from the transmission apparatus 101 to the transmission apparatus 103 can be transmitted through only a route passing through the transmission apparatus 104 while securing a backup channel. Consequently, the line 101b connected to the transmission apparatus 102 can be set so as to transmit another transmission signal.
For example, the transmission signal to be relayed and transmitted to the transmission apparatus 102 can be transmitted through the line 101b together with the transmission signal for the transmission apparatus 103.
Here, in the case where a fault occurs in the line 103a between the transmission apparatus 103 and 104 as shown in FIG. 38(b), unlike the above-mentioned UPSR in which the faultless path is selected by the path switch 103A in the transmission apparatus 103 on the receiving side, APS (Automatic Protection Switch) protocol is used in the BLSR so as to perform line loop back, thereby enabling relief.
Namely, as shown in FIG. 39, when the transmission apparatus 103 detects an occurrence of fault in the line 103a, it performs a protection action, whereby the signal inputted from the transmission apparatus 101 through the optical fiber 105-2 is turned around within the transmission apparatus 104 (bridging processing) so as to be outputted to the transmission apparatus 103 through the optical fiber 105-1. That is, the transmission apparatus 104 operates as a switching node.
In the above-mentioned transmission apparatus 104, when a transmission signal is transmitted to the transmission apparatus 103 through the long path, transmission line fall information (SF-RING; Signal Fall) is assigned, as a request, to K1 and K2 bytes of SOH (Section Overhead) of the SDH frame constituting the transmission signal.
In the case where a fault occurs in the line 103a, the transmission signal transmitted to the transmission apparatus 101 is transmitted to the transmission apparatus 103 through a route, as a long path (bypass route), passing through the transmission apparatus 101 and 102 (loop back) The transmission apparatus 101 and 102, as these intermediate nodes, are in the state of full pass-through, whereby K bytes and protection channel signals pass therethrough as they are.
When the transmission signal is inputted into the transmission apparatus 103 via the long path, as this signal is subjected to a switching operation, the signal from the long path can be selected (switching operation) instead of the signal from the short path (signal from the transmission apparatus 104), so as to be relayed to another line 103b. Namely, the transmission apparatus 103 also operates as a switching node.
Thus, by performing the loop back using APS protocol, the BLSR improves the reliability of networks while yielding a line utilization efficiency higher than that of the above-mentioned UPSR.
As the transmission capacity is currently increasing, various kinds of needs for the use of thus increased capacity are expected to emerge. Namely, in order to satisfy such needs, required is a transmission apparatus which can construct, in response to a request of a person constructing a network, a ring network having a plurality of transmission systems different from each other.
Namely, demanded is a transmission apparatus which, while employing the same transmission apparatus as a constituent of ring networks, can construct not only a ring network which can operate as UPSR, but also a ring network which can operate as BLSR.
For example, as shown in FIG. 40, by connecting together a ring network 111 as UPSR, a ring network 112 as BLSR, and a ring network 113 as UPSR, such transmission apparatus 110 can efficiently construct a network which can operate, as a whole, as a UPSR ring network 114, while keeping a performance of BLSR in the ring network 112. Accordingly, the degree of freedom in design when constructing networks can be expected to improve.